The present invention is related generally to the field of locating using an electromagnetic signal and, more particularly, to locating relative to a path using an electromagnetic locating signal. The apparatus and method of the present invention are highly advantageous with regard to determination of orientation relative to a target borehole, for example, in an operation intended to form another borehole arranged having a particular orientation with respect to the target borehole.
A number of approaches have been taken in the prior art with regard to locating relative to a path using an electromagnetic locating signal. The predominant application has been seen in the field of underground locating for the purpose of forming a borehole that is parallel, at some desired offset, from a pre-existing borehole. Such parallel boreholes are generally used for the purpose of enhancing extraction of heavy oil reserves. The pair of boreholes includes at least one horizontally spaced-apart section positioned to extend through the heavy oil reserve. Steam is generally injected into one of the parallel pair of boreholes forming an uppermost portion of the horizontally extending section serving to heat and thin the oil surrounding it. The other borehole comprises a lowermost portion of the horizontally extending section which receives the heated and thinned oil for recovery.
One approach to the problem of forming a borehole, that is drilled in relation to an existing, target borehole (itself defining a path for locating relative thereto) is seen in a family of patents issued to Kuckes et al. including, as an example, U.S. Pat. No. 5,485,089. A common feature throughout these patents resides in the use of a “solenoid” to transmit a point source, dipole locating signal from the target borehole which varies in three dimensions emanating from the point source. As will be described below, this feature is considered as being disadvantageous based on signal decay characteristics and in view of further discoveries that are brought to light herein.
A more general approach for use in guiding a drilling operation is seen in U.S. Pat. Nos. 3,529,682 and 3,712,391 issued to Coyne (hereinafter the Coyne patents). These patents describe a guidance system for guiding a mole, for example, a drill head, with respect to a pair of antennas that is laid out on the ground. While the Coyne patents describe an elongated axis antenna capable of being positioned along a path, the advantages of the Coyne patents are inextricably founded upon the use of a rotating magnetic field detector received at the location of the mole. This relatively complex field vector is produced using a dipole-quadrupole antenna that is actually made up of two separate antennas. Specifically, what the '391 patent describes as a dipole antenna is a wire loop which itself surrounds a quadrupole antenna. This antenna pair must be driven in a specialized manner to produce the desired field characteristic. As a first example, each one of the pair of antennas is driven by a separate, out-of-phase signal. As a second example, the antenna pair may be driven with two distinct frequencies or with at least some sort of identifiable timed variation between the two signals that drive the two antennas. In any case, the rotating field vector must be produced.
While the disclosure of the '391 patent states that any suitable antenna may be used to produce a preferred, circularly polarized locating signal, the disclosure favors the use of these two antennas, in combination, for reasons of its “simple geometric relationships” (col. 2, ln. 6-7). As will be further described at an appropriate point hereinafter, the use of a rotating flux vector is considered as unduly complex and burdensome in light of the teachings of the present invention.
The present invention resolves the foregoing disadvantages and difficulties while providing still further advantages, as will be described below.